This disclosure generally relates to infrared detectors, and particularly relates to field-assisted infrared detectors with unipolar barriers.
Photodetectors are electro-optical devices that respond to incident electromagnetic radiation. A photodetector sensitive to infrared wavelengths of light is also known as an infrared (IR) detector. IR detectors are used in a wide variety of applications including thermal detection for surveillance, tracking, night vision, search and rescue, non-destructive testing and gas analysis. Typically, an IR detector is formed as a device consisting of an array, usually rectangular, of IR-sensing photodetectors disposed at the focal plane of an imaging lens. Such a detector is commonly referred to as a focal plane array (FPA).
Modern IR photodetectors are often produced using InSb and HgCdTe (mercury, cadmium, telluride (MCT)) materials fabricated as p-n junction diodes. However, InSb photodetectors based on p-n junctions require low operating temperatures due to generation-recombination (G-R) current from the p-n junction and surface generation current from passivation, and MCT photodetectors suffer from poor uniformity and correctability resulting in a distribution “tail” of lower performing pixels. MCTs are also limited due to the lack of commercially available, large format substrates. Although the use of cryogenic temperatures can reduce the dark current generated in, for example, the p-n junctions of the bulk materials and at the surface of the material by Shockley Reed Hall (SRH) generation, this approach is complex, expensive, and imposes serious size and energy limitations on the resulting devices.
Dark current affects many photosensitive devices and is characterized by a relatively small electric current that flows through the device even when no photons are entering the device. Dark current is one of the main sources of noise in imaging detectors such as IR detectors, and has traditionally been mitigated by operating the detectors at temperatures significantly below ambient (room) temperature as described above. Dark current occurs due to the random generation of electrons and holes within the device. Photodetectors employing p-n junctions are especially prone to generation of dark current due to low activation energies in the depletion region of such detectors.
IR detectors based on III-V alloys are an attractive alternative to MCT-based photodetectors because of the large commercial III-V infrastructure and the availability of high quality, large format substrate materials. Barrier photodetectors comprising a photo-absorbing layer, a barrier layer, and a contact layer have been developed that can tolerate significantly higher operating temperatures. One example of a photodetector using the barrier structure above is referred to as an nBn detector.
An nBn photodetector containing a unipolar barrier layer can be engineered in various ways to absorb a target IR waveband. The term “unipolar” means that the barrier that can block one carrier type (electron or hole) while allowing relatively unimpeded flow of the other carrier type. For example, these detectors may use a barrier layer whose minority carrier band edge lines up with the absorber minority carrier band edge so that carrier can be collected. The majority carrier band edge of the barrier is well above the contact or absorber band edge such that majority carriers are blocked or filtered—thus producing a so-called “majority carrier filter.” In this construction the thickness of the barrier layer is sufficient to prevent tunneling of majority carriers from the photo-absorbing layer to the contact layer, and a barrier in the majority carrier energy band is sufficiently thick to block the flow of thermalized majority carriers from the photo-absorbing layer to the contact layer. Importantly, the barrier layer is engineered not to significantly block minority carriers when an appropriate bias voltage is applied.